My guess is somebody (assuming you're not the original owner) gutted the cat because they thought they'd get more power, then maybe put o2 simulators so there wasn't a CEL.
OR, the cat broke down over time and the down stream O2 sensor is bad, thus making the cat monitor think everything is ok.
"Anything to do with the land, I love."
"So it's probably eighty percent luck and twenty percent skill."
I never could figure what the sky was thinking, but the soil, she don't keep too many secrets.